1. Field of the Invention
The present invention relates to a method for manufacturing a printed-wiring board and, in particular, to a method for manufacturing a printed-wiring board having a resistive element therein.
2. Related Art
In recent years, with miniaturization and high functionality of an electronic device, the mounting density of components has remarkably increased. Accordingly, a printed-wiring board having built-in components with increased mounting density has been studied, in which passive components are formed on an inner surface of the printed-wiring board, which will replace a conventional method: soldering passive components such as resistors and capacitors on a printed-wiring board in a chip-component form.
A method of forming resistors of passive components on an inner surface of a printed-wiring board has been conventionally put to practical use through a ceramic multilayer printed-wiring board. However, because a resistor is formed by screen printing, the resistor has various resistance values. After firing of a resistor, a desired resistance value must be obtained by trimming the resistor with laser or sand-blasting it. The firing temperature is high, above 500° C. and cannot be applied to an organic resin printed-wiring board.
Moreover, as a trial to an organic resin printed-wiring board, there have been studied a method for forming a resistive thin film on the whole surface and obtaining a desired resistance value by etching and a method for applying low-temperature fired resistive paste by screen printing to obtain a desired resistance value. The resistors formed on inner surfaces of the printed-wiring boards are applicable to required wide variety of resistance values, and few variations in resistance value, that is, high accuracy in preparing a resistor pattern and a uniform resistive film thickness are required.
The above-described thin-film method can achieve high resistor pattern accuracy; however, the range of an obtained resistance value is narrow because of a thin film. On the other hand, a resistive paste method provides wide variety of resistance values; however, it is inferior in accuracy of a resistor pattern formed by screen printing and uniformity of film thickness. Accordingly, the resistive paste method requires laser trimming to increase accuracy of a resistance value.
It has been known that the resistors formed by the resistive paste method suffer from the resistance value changes, when undergoing laminating press in order to incorporate them in a printed-wiring board. A variation in the resistance value varies depending upon laminating conditions, type of laminating adhesive agent or film thickness and size of a resistive paste and therefore is difficult to previously estimate the variation of the resistance value due to laminating press before trimming and obtain a desired resistance value after lamination.
The printed-wiring board with built-in resistive element disclosed in Japanese Patent Laid-Open No. 2006-222110 (refer to [0006] at page 3) prevents a variation in a resistance value under high temperature and humidity by forming a substitution type non-electrolytic silver plating film between resistive paste and an electrode. However, the method cannot suppress a variation in a resistance value due to lamination.
Moreover, the printed-wiring board with built-in resistive element disclosed in Japanese Patent Laid-Open No. 2004-335827 (refer to [0012] at page 3 to [0013] at page 4) has improved a trimming method for resistance value adjustment and can adjust the resistance value to a lower or higher one. However, an adjustment method cannot be determined until a resistance value has been measured, which causes a troublesome process. Further, no consideration is given to a change in a resistance value due to a laminating process after adjustment of the resistance value.
The printed-wiring board with built-in resistive element disclosed in Japanese Patent Laid-Open No. 2000-174405 (refer to [0011] at page 2) can make the printed-wiring board itself serve as a spacer to restrain variations in resistance values due to laminating because a through hole is filled with resistive paste. However, simply filling the hole with paste cannot create a highly accurate resistance value and therefore there occurs such a structure that resistance value adjustment cannot be made even by laser trimming.
In the printed-wiring board with built-in resistive element disclosed in Japanese Patent Laid-Open No. 2006-228781 (refer to [0005] at page 3), a resistor is heated by laser with lowered power, when trimming to adjust a resistance value, to change the resistance value and subsequently to perform trimming processing with increased power.
However, to heat the resistor, the whole area of the resistor must be scanned with weak laser beams and therefore it takes much time. Variations in resistance value at laminating are not caused simply by heating and there is an influence of laminating pressure. This method cannot complete variations in resistance value.
In the printed-wiring board with built-in resistive element disclosed in Japanese Patent Laid-Open No. 2006-156746 (refer to [0020] at page 5), a nickel alloy thin film is formed between resistive paste and an electrode to prevent variations in resistance value under high temperature and humidity. However, the method cannot restrain variations in resistance value due to laminating.
FIG. 2 is a sectional view illustrating a method for manufacturing a printed-wiring board with built-in resistive element using resistive paste disclosed in Japanese Patent Laid-Open No. 2006-156746, where there is prepared a double-sided copper-clad laminate 31 having a first and a second conductive layers of copper foil or the like on both sides of an insulating base material such as polyimide to form a through-hole at a required position by drill processing or laser processing.
Subsequently, a plating film is formed by conductive processing and a circuit pattern is formed using an etching process by usual photo-fabrication to obtain a double-sided printed-wiring board 32.
Next, a nickel alloy thin film 33 is formed on an electrode portion in contact with resistive paste and a resistor 34 by the resistive paste is formed by screen printing between the electrodes covered with the nickel alloy thin film 33. Next, a resistance value of the resistor 34 is adjusted by forming a removed portion by laser trimming or the like.
Subsequently, four-layer structure is formed by laminating copper foil 36 with resin or the like. Subsequently, a bottomed via-hole for performing inter-layer conduction is formed by laser and plating coating is formed by conductive processing. Subsequently, a circuit pattern is formed using an etching process by photo fabrication, thus obtaining a printed-wiring board 37 with built-in resistive element.
A terminal resistor of a transmission line or EMI filter resistor needs a technique of inexpensively creating resistance value accuracy of ±1 or less, in a built-in state on a printed-wiring board.
However, a conventional built-in resistive element formed by resistive paste cannot accommodate resistance changes before and after lamination and therefore it is difficult to perform high-yield manufacture with accuracy of ±1 or less, in a built-in state.